docs(build): correct OOM-daemon guidance and add global cargo/systemd knobs

systemd-oomd is desktop-only on Ubuntu (reverse-deps are ubuntu-desktop*);
'systemctl is-enabled systemd-oomd' returns not-found on this server, so
the previous advice to enable it would simply have failed. Recommend
earlyoom instead, with --avoid/--prefer tuned to protect the shell and
target rustc.

Also document the machine-wide alternatives to per-project tuning:
- sudo systemctl set-property user-1000.slice MemoryHigh=/MemoryMax=
  (covers every shell and build; sshd lives in system.slice so it can
  never lock you out)
- global ~/.cargo/config.toml, which does support [profile.*]

Clarify that the MemoryHigh warning applies to short-lived build scopes,
not to slice-level limits where gradual throttling is appropriate.

docs/BUILDING.md

@@ -110,16 +110,43 @@ The machine-level protections below need no per-project changes.
 ```bash
 # Reserve memory for the SSH daemon so it can't be swapped out entirely.
 sudo systemctl edit ssh        # [Service] / MemoryMin=128M
-
-# Kill on memory *pressure* rather than waiting for true OOM. This reacts
-# during thrashing — exactly the window where the box currently becomes
-# unreachable. Ubuntu ships it but leaves it inactive.
-sudo systemctl enable --now systemd-oomd
 ```
 
-`earlyoom` is the lighter-weight alternative (`sudo apt install earlyoom`);
-it kills only the single highest-scoring process, which during a build is
-`rustc` itself.
+**Do not bother with `systemd-oomd` on this box** — it is a separate
+package whose only reverse-dependencies are `ubuntu-desktop*`, so Ubuntu
+Server never installs it (`systemctl is-enabled systemd-oomd` → `not-found`
+here). Use **earlyoom** instead, which is in `universe` and kills only the
+single highest-scoring process rather than a whole cgroup:
+
+```bash
+sudo apt install earlyoom
+# /etc/default/earlyoom
+EARLYOOM_ARGS="-m 8 -s 5 -r 60 \
+  --avoid '(^|/)(systemd|sshd|mosh-server|tmux.*|bash|fish)$' \
+  --prefer '(^|/)(rustc|cargo|ld|lld|collect2)$'"
+```
+
+`--avoid` protects your shell; `--prefer` points it at the compiler. Fedora
+enabled earlyoom by default for exactly this "system becomes completely
+unresponsive, user has no choice but to force power off" scenario.
+
+### Cap every build at once, forever (needs root once)
+
+This is the global knob — no wrapper script, no per-project config:
+
+```bash
+sudo systemctl set-property user-1000.slice MemoryHigh=5G MemoryMax=6500M
+```
+
+Applies to every shell and every build you start, persists across reboots.
+Because `sshd` itself lives in `system.slice`, capping the user slice can
+never lock you out of a new login. (Ubuntu already ships `TasksMax=33%`
+this way via `/usr/lib/systemd/system/user-.slice.d/`, so it's a
+distro-blessed pattern.)
+
+Note `MemoryHigh` at the *slice* level is reasonable — it throttles a
+sprawling session gradually. Do not put it on a single short-lived build
+scope, where it causes the reclaim-stall described below.
 
 ### Cap one build ad-hoc (no root)
 
@@ -136,10 +163,11 @@ OOM — it was *thrashing*. With swap available the kernel pages `sshd` out
 to feed the build and the box goes catatonic while `oom_kill` stays at 0.
 Denying the build swap turns a slow total failure into a fast contained one.
 
-**Do not add `MemoryHigh`.** It sounds safer but traps the process in
-continuous reclaim so it grinds forever instead of dying. Measured against
-an identical 256 MB ceiling: `MemoryMax` alone → clean kill in seconds;
-`MemoryMax` + `MemoryHigh` → still spinning after 45 seconds.
+**Do not add `MemoryHigh` to a single build scope.** It sounds safer but
+traps the process in continuous reclaim so it grinds forever instead of
+dying. Measured against an identical 256 MB ceiling: `MemoryMax` alone →
+clean kill in seconds; `MemoryMax` + `MemoryHigh` → still spinning after 45
+seconds.
 
 ### Diagnosing which crate is expensive